An organic EL element serving as a current driving type display element has such a well-known property that luminance depends on a current value, and that duration is short when the organic EL element is driven by a large current for attainment of a high luminance display. Developed for acquirement of a wider display screen and high definition in a display apparatus including such an organic EL element is an active matrix driving. Conventional passive matrix driving suffers from a difficulty in attainment of high luminance due to an increase in the number of scan lines, and from a decrease in duration due to momentary application of a very large current to pixels. For this reason, the passive matrix driving is implemented for relatively short-life use.
Incidentally, big problems of the active matrix driving are (i) current non-uniformity due to property variation of a thin film transistor (TFT), and (ii) uneven display luminance due to threshold voltage non-uniformity thereof. Other problems are (i) a decrease in luminance due to a deterioration of the organic EL with age, and (ii) a change (as temperature rises, the luminance rises) in luminance due to light emission (heat emission) of the organic EL. Now, a function for compensating such adverse properties has been required.
In order to solve the problems, various conventional driving circuit methods have been proposed. Examples of such methods include: (i) a voltage program method disclosed by Document 1 (WO98/48403, published on Oct. 29, 1998) and (ii) a current program method disclosed by Document 2 (WO01/07582; published on Oct. 11, 2001).
FIG. 20 is a circuit diagram illustrating a structure of a pixel circuit driven in accordance with the voltage program method. The pixel circuit shown in FIG. 20 is driven such that an analog voltage is applied from a data line 310 to the pixel circuit. With this, an output current of a transistor 365 (driving TFT) is programmed.
In the analog voltage program method, an initializing voltage (reference voltage) is applied from the data line 310 to a terminal of a capacitor 350, which terminal is toward a transistor 360 (switching TFT). This turns ON a transistor 370 (switching TFT), a transistor 375, and the transistor 365. Thereafter, the transistor 375 is turned OFF, and a threshold voltage correction is carried out with respect to the transistor 365. The threshold voltage correction requires several ten microseconds. Thereafter, the transistor 370 is turned OFF, and a desired voltage is applied to the terminal of the capacitor 350. With this, the output current of the transistor 365 is determined.
Because the threshold voltage variation in the respective transistor 365 is compensated as such, a constant driving current controlled according to the data voltage is supplied to an OLED 380 irrespective of the threshold voltage of the transistor.
Meanwhile, FIG. 21 illustrates a circuit diagram illustrating a structure of a pixel circuit driven in accordance with the above current program method. The pixel circuit is driven as follows. That is, a potential of a gate wire 42 is set at Low so as to turn ON transistors 32 and 37 (switching TFTs), and so as to turn OFF a transistor 33 (switching. TFT). Then, a current is supplied from a transistor 30 (driving TFT) to a row driving circuit (not shown; source driver) via a source wire 44. This allows a setting of a gate voltage of the transistor 30, and accordingly allows a setting of an output current of the transistor 30.
Thereafter, the gate wire 42 is set at High, and the transistors 32 and 37 are accordingly turned OFF, with the result that the gate voltage of the transistor 30 is maintained. Then, the transistor 33 is turned ON, and the current thus set is supplied to an organic EL 20.
Such a current program method allows compensation of (i) the threshold voltage variation of the transistor 30, and (ii) mobility variation of the transistor 30.
However, the driving method in Document 1 requires 60 microseconds or longer for the writing in each pixel. Supposing that a display is carried out with the use of a QVGA format (240×320 pixels) compliant display apparatus of portrait type (320 lines are vertically provided), and that a single frame period corresponds to 1/60 second, the writing in each pixel has to be carried out in 1/(320×60) second≈52 microseconds.
As such, the pixel circuit (see FIG. 20) takes time for the threshold correction of the driving TFT, and a display therefore cannot be attained in the required number of the pixels.
On the other hand, the current setting method disclosed by Document 2 also suffers from such a problem that the setting of the output current of the transistor 30 takes long time. Specifically, the source wire 44 has normally has a stray capacitance of several pF. Supposing that the stray capacitance is 10 pF and that a current value set for the transistor 30 is 0.1 μA, it takes 0.1 ms to change by 1 V, the voltage of the source wire 44. The threshold value of the transistor 30 of each pixel varies by on the order of 1V, so that the setting of the output current value requires 0.1 ms or longer.
Thus, the analog voltage driving method (see FIG. 20) and the analog current program method (see FIG. 21) requires such a long time for the setting of the output current from the driving TFT, so that a display cannot be attained in the required number of display pixels.
Such a problem is especially noticeable upon carrying out a time-division gradation display. In other words, for the acquirement of the time-division gradation display, the current setting is required to be carried out, within one frame period, with respect to transistors in pixels whose number corresponds to the number found by multiplying the gate wires by sub-frames.